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  • Hard is Good

    From this posting, it will be very obvious that my knowledge of metallurgy is VERY limited. I have heard that some steels are more difficult to harden than others, but I'm not sure of just what that means. Does it mean that they can be hardened but with greater difficulty, or does it mean that they simply cannot be hardened. What are the chances of my reaching into a pile of unknown steel, and coming up with a piece that cannot be hardened? If it can be hardened, how do I know if it should be quenched in oil, air, or water? Assuming that it can be hardened, can it be done using a propane torch, or is that enough heat? As I said at the beginning, I plead total ignorance. I do know that this is a complicated subject and I don't expect a college course, but if it could be generalized to some extent. that would help, and I suspect others as well.
    There is no shortage of experts, the trick is knowing which one to listen to!

  • #2
    Generally speaking, if you have to make a part that has to have a specific hardness then it would be best to buy stock that can be hardened to that characteristic and meet the other criteria needed as well such as machinebility and strength. Without knowing what kind of steel you may have, there is no way of prescribing a proper heat treating procedure. If you are just making a small widget where strength or hardness doesn't matter then as long as you can cut it with a file and can manage to make it with your machines then have at it.
    Jonathan P.

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    • #3
      "What are the chances of my reaching into a pile of unknown steel, and coming up with a piece that cannot be hardened?"

      Most all steels can be hardened in some way or another.

      I am also limited to what I know about it. What I do is stay away from the scrap piles. It is very tempting to grab something that looks like it might do the job only to find out that you can't even drill a simple hole in it. Or worse.

      When I go to buy metal at my local steel supplier, I do look in their scrap bins, BUT, I only pick out the stuff that still has some kind of color code on it. I then can ask the guy behind the counter what the heck is this?

      Saves a lot of frustration.

      There are ways to determine what a piece might be by touching it to a grinding wheel, and there are folks hear that can do that. It would be nice to learn maybe. But for now I use metal that I know for sure what it is.

      Another thing I do once I find out for sure what the stuff might be I always keep it well maked in case I happen to have to cut off the color code (that I can't keep track of anyway). Tape or magic marker works for me.

      Good questions.
      Gene

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      • #4
        I'm no expert by any means but I'll make come generalized comments. Steel that is "heat treatable" means that it will respond in a predictable manner to a specific regimen of heating and cooling. In order to fall into this category, it must have had certain alloying materials added when the steel was made. The most common is the amount of Carbon, but other alloys such as Nickel, Chromium. etc are all commonly used. If the steel is "low carbon", then it probably will not repond to attempts to harden or "heat treat" as there is not enough Carbon in it to respond to the process. I believe Carbon can be added to produce a "surface hardness" using a product like Kasenite.

        Anyhow, the basic idea is that at elevated temperatures the Carbon is free to move about within the alloy and form differnt types of crystalline structures - some of which have desireable physical properties. When the material is elevated to a specific temperature and held there for a specific time, a known structure is developed as the Carbon distributes itself and forms these molecular structures. The task is then to cool the steel rapidly so as to lock these structures in place. If it is allowed to cool slowly, the carbon will revert back to its non desireable structures (unless you are trying to anneal steel that is already hard).

        Basically you can use some of the "spark tests" shown in many books to get a crude idea of whether your steel has enough Carbon in it to repond to heat treat methods. For a predictable result, you must know the alloy of the material and follow the guidelines for heating and quenching. I don't think propane burning in "free air" will produce a high enough temperature for heat treat and some steels need to be treated in an inert environment. When I work with tool steel, I buy it annealed, machine it, and then take it to a heat treat place and tell them the Rockwell hardness I want (usually the full hardness that the alloy will develop). They know exactly how high to heat it, for how long to hold it there (usually hours), what the environment must be during the elevated temperature, and how to cool it. I think sometimes they are quenched in a heated oil bath, sometimes a heated salt solution, sometimes water, and sometimes air cooled. That is their specialty.

        Comment


        • #5
          topct is right, how you'd go about it though depends on what the alloy is, or what the steel is made of and there is more to selection of a steel than 'can it be hardened or not'. without the addition of more alloys (ie case hardening) the basic answer to your question on whether it can be hardened is is how much carbon there is but that won't mean anything to you without some knowledge. here's some help on getting a base level of metallurgy:

          http://72.14.207.104/search?q=cache:...n&ct=clnk&cd=2

          also

          go to http://www.mcmaster.com/ look under materials, metals, steel - there is a decent overview of common alloys there

          is this a general knowledge question or practical, ie what is it your are trying to do?
          .

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          • #6
            I would strongly recommend this book.

            I have it and it is excellent as are all the books by Tubal Cain.
            Free software for calculating bolt circles and similar: Click Here

            Comment


            • #7
              What Evan said...this is a subject that needs a book (or several books) to explain adequately. I second his recommendation of almost anything Tubal Cain has ever written.

              VERY briefly, to a first approximation, the magic alloying element the permits steel to harden is carbon. Below about 0.4% carbon, there isn't enough for the steel to harden; you can heat and quench it all day and it still won't get hard.

              Above that level, if you heat the steel above its critical temperature (which happens to be the point at which the steel will no longer be attracted to a magnet) and then cool it suddenly, e.g. by quenching it in water, the steel will become hard, and also brittle, to the point that it may shatter if you try to do anything with it. Typically, steel is tempered after hardening by heating to about 300-400 degrees F to reduce the brittleness. This also reduces the hardness somewhat, but enough hardness remains so the steel is still "hard" and enough of the brittleness is eliminated so the steel will not crack in use.

              The tempering process is where the "colors" come in you may have heard about. If a hardened piece of steel is polished, then heated, the polished area will turn light yellow (straw), then darker yellow, turning to blue, purple, and brown as it gets hotter and hotter. These colors correspond to temperature, so one can judge the amount of tempering by the color, at which point it's quenched again.

              That's REALLY basic. As others have said, there are other alloying elements that can affect the hardening, to the point that "air hardening" steel will harden if heated and allowed to cool in air. The quenching medium used (water, a brine solution, oil, or whatever) controls the rate of cooling, which also affects how the steel hardens.

              Oh - somebody mentioned Kasenit. If you heat a low-carbon steel (i.e. less than 0.4%) in a high-carbon environment typically provided by covering it with a carbon-rich powder (Kasenit is one commerical brand) or enclosing it in a carbon-rich material while the steel is heated, the outer skin of steel will absorb enough carbon to make it "high carbon" steel. Then, when quenched, the outer skin will become hard even though the inside of the part remains soft. This can provide a wear layer on a part, and also can allow a very high surface hardness because the part doesn't need to be tempered after hardening -- the soft core of the part will prevent cracking.
              ----------
              Try to make a living, not a killing. -- Utah Phillips
              Don't believe everything you know. -- Bumper sticker
              Everybody is ignorant, only on different subjects. -- Will Rogers
              There are lots of people who mistake their imagination for their memory. - Josh Billings
              Law of Logical Argument - Anything is possible if you don't know what you are talking about.
              Don't own anything you have to feed or paint. - Hood River Blackie

              Comment


              • #8
                Quick lesson in reading alloy numbers. 1018 steel is 0.18% carbon. 1040 is 0.40% carbon. The last two digits in the SAE-AISI designation system indicates the carbon content in hundredths of a percent. 4130 is 0.30% and 4140 is 0.40% etc.

                SGW, it is about 0.30% where the cutoff for hardenability is. 4130 hardens just fine, especially when you don't want it to when gas welding it.
                Free software for calculating bolt circles and similar: Click Here

                Comment


                • #9
                  I think that may be because 4130 has additional alloying elements in it...I'm pretty sure plain basic carbon steel needs about 0.40%...but I'm perfectly willing to be wrong!

                  Anyway, the basic point is that there IS a minimum, whatever it is, and unless it's there the steel won't harden.
                  ----------
                  Try to make a living, not a killing. -- Utah Phillips
                  Don't believe everything you know. -- Bumper sticker
                  Everybody is ignorant, only on different subjects. -- Will Rogers
                  There are lots of people who mistake their imagination for their memory. - Josh Billings
                  Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                  Don't own anything you have to feed or paint. - Hood River Blackie

                  Comment


                  • #10
                    Here is some info from Key-to-Steel.

                    Steels with 0.25 to 0.55% C. Because of their higher carbon content, these steels are usually used in the hardened and tempered condition. By selection of quenching medium and tempering temperature a wide range of mechanical properties can be produced. They are the most versatile of the three groups of carbon steels and are most commonly used for crankshafts, couplings, tie rods and many other machinery parts where the required hardness values are within the range from 229 to 447 HB. This group of steels shows a continuous change from water-hardening to oil-hardening types. The hardenability is very sensitive to changes in chemical composition, particularly to the content of manganese, silicon and residual elements, and to grain size; the steels are sensitive to section changes.

                    http://www.key-to-steel.com/default....kArticle&NM=90
                    Free software for calculating bolt circles and similar: Click Here

                    Comment


                    • #11
                      Hardenability....

                      Further Opinions.....
                      Jim C. said alot about basic hardenability requirements. I will also recommend Tubal Cains book(s), they are all good.
                      As Evan, Topct & others have said, much has been written about heat treatment of metals. It is an old & very diverse art & science....
                      I wouldn't depend on being able to harden a piece of mystery steel in any predictable way. The grinder spark test can give you a general indication of what it might be....but....
                      When I need to heat treat something, I like to start with a known material. Low carbon "mild steel" eg. 1018....can be case hardened with Kasinite or in a cyanide salt bath for some purposes.
                      I have a small electric furnace & a gas furnace, so I do some heat treating myself. If it's a part for a customer that needs close control, it goes to a reputable heat treat firm....
                      Just my opinions.
                      Rick

                      Comment


                      • #12
                        I will slide this one in here rather than start another topic.

                        Aluminum.

                        I have in front of me three pieces of round stock. 6061, 2024, 7075. I would dare anyone on the face of the earth to tell me what each of them are by looking at them. It's only because they are stamped with thier numbers that I can tell.

                        But each one of them has it's own properties, and what and where they might be used for. And how they machine.

                        Brasses are another, and so are the various bronzes. All the metals we work with.

                        The amount of information available to everyone as to the metals we are able to work with, and their applications, is astounding. There are no secrets.

                        It's just so not neccesary to work with junk (well maybe not junk but with something we don't even know what it is) that amazes me why people would bother.
                        Gene

                        Comment


                        • #13
                          You know what? It's in your BIBLE. The time it takes waiting for replies, you could have read heat treating.

                          Comment


                          • #14
                            A local shop gave me some hardening powder- don't know what it was called, but it could have been kasenit. I drilled a kind of deep dimple into a piece of steel, then mounted that on a stable base. Nearly filled the dimple with the powder, then heated my part to red and ground it around in the powder. Reheated the part, ground it around in the powder again. Repeated this 3 or 4 times, then heated to the non-magnetic point and quenched. I tried drill rod, spikes, and other scrap bits. All came out so hard that a file would skate across rather than cut.

                            I've made many useful cutting tools from drill rod, but I've never tried to use a spike for that purpose. If the project at hand requires little more than a hard surface on it, you can probably get away with using mystery metal. As others have said though, if the part needs to have predictable qualities and some minimum strength and toughness, then the proper material should be used from the start. If the part will be stressed considerably in use, and/or if the part breaking while in use poses a danger, then possibly you shouldn't risk heat treating it yourself, even if you know the material or choose the right material for the job.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                            Comment


                            • #15
                              Found out a couple of years ago that 1018 could be hardened. If heated and quenched properly the microhardness of the surface is increased slightly. NOT enough to detect with a file or a common hardness tester, but enough to increase the life of a shaft in a sleeve bearing. Hard to control in a production setting and therefore not often used.

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